A conventional composite filter used in portable wireless communication devices such as mobile phones is described with reference to FIG. 9, which shows its perspective exploded view. In FIG. 9, the conventional composite filter is formed of a combination of an elastic surface-wave filter and a low-pass filter.
The low-pass filter is formed by combining an inductor with a capacitor, and is embedded in laminated substrate 2 made of low temperature co-fired ceramics and formed by layering a plurality of dielectric layers 1. The inductor is formed of a plurality of dielectric layers 1 with arc-shaped conductors 3 formed thereon and coupled to each other. The capacitor is formed of planar capacitor electrodes 4 sandwiching dielectric layer 1.
The elastic surface-wave filter is formed by combining elastic surface-wave resonators 5 in a ladder shape as shown in FIG. 10, and placing resonators 5 on laminated substrate 2.
The low-pass filter is coupled to a transmission output side of the foregoing filter in order to attenuate the doubled or tripled harmonics in the pass band of the filter. The composite filter discussed above, however; is formed by embedding the low-pass filter in laminated substrate 2, so that the inductor and the capacitor are obliged to occupy a large area, and the capacitor, in particular, takes a greater portion of the occupied area because its capacitance is proportionate to its area. As a result, the filter cannot be further downsized.
The art related to the present invention is listed here as patent documents 1 and 2.
Patent Document 1: Unexamined Japanese Patent Publication No. 2004-254257
Patent Document 2: Unexamined Japanese Patent Publication No. 2005-184773